1. Technical Field
The invention relates to intrusion detection systems and in particular to intrusion detection systems which comprise an open or "leaky" transmission line, for example a leaky cable, for receiving a radio frequency signal and receiver means attached to the open transmission line for processing the received radio frequency signal to detect perturbations caused by an intruder in proximity to the open transmission line.
2. Background Art
It is desirable for such an intrusion detection system to have uniform sensitivity along the length of the open transmission line. All transmission lines, including open transmission lines, attenuate signals as they propagate along their length. This attenuation is due to resistive losses in the conductors, to losses in surrounding dielectric materials, or to radiation. Usually attenuation is dominated by resistive losses. With a line receiver connected to one end of the line, and the detector sensitivity set to detect a large intruder, for example a human, at the far end of the line, the alarm could be triggered by a small animal, for example a rodent, adjacent the near end of the line. In systems employing a leaky coaxial cable as the line, this problem has been addressed by increasing the aperture size with distance along the cable (cable grading). Such graded cables are very expensive, which is a disadvantage for large area protection, and must be supplied and stocked in standard lengths, which leads to additional expense.
U.S. Pat. No. 4,224,607 (Poirier) issued Sep. 23, 1980, discloses an intrusion detection system comprising a length of leaky transmission line encompassing the region to be protected, a receiving antenna within the region, an R.F. transmitter and a remotely located receiver and detector. Effects of attenuation along the line are reduced by switching the transmitter periodically to opposite ends of the leaky cable. Nevertheless, this is only a partial solution. The sensitivity is still greater at the ends of the cable than at the middle of the cable so the system would not necessarily provide detection anywhere along the cable while not being susceptible to false alarms from the more sensitive start end of the cable. Where a longer line is needed to protect a greater area, especially if an inexpensive cable with a relatively high attenuation is used, the variation in sensitivity would be even greater.